1. Field of The Invention
The invention relates to a method for operating a high-temperature fuel cell, which in normal mode of generating electrical power is supplied with liquid fuel, preferably diesel oil, and is preceded by a reformer for Liquid fuel on the anode side, where at least part of the hot anode exhaust gas is recirculated into the anode circuit via a recirculation line, and where upstream of a compressor preceding the reformer the liquid fuel is sprayed or injected into the hot anode exhaust gas, the quantity of air needed for reforming the liquid fuel being added to the mixture of anode exhaust gas and fuel.
1. The Prior Art
A high-temperature fuel cell, for instance a solid-oxide fuel cell (SOFC) with an anode circuit for recirculating water for the reformer process, and a method for operating a fuel cell of this kind, are disclosed in AT 502.130 B1, for instance, This method permits operation of fuel cells without the supply of additional external water. Water is required for endothermal steam reforming of hydrocarbons. If liquid hydrocarbons such as gasoline or diesel are used, the reformer process can be substantially improved by means of the recirculated water and the operating temperature can be lowered.
The operational profiles of SOFC systems differ according to application. In stationary applications the systems are permanently operational. In mobile applications, for instance in auxiliary power units (APU) of heavy duty trucks, the systems are operational only part of the time and must be kept at a certain temperature for the rest of the time. This phase, during which no current is generated, is highly critical, since the anode of the SOFC must be kept in a reducing environment (with fuel), but no water for reforming is available. This same problem arises not only in standby operation but also during start-up and shut-down of the fuel cell.
Deposits and contaminations in the reformer and the fuel cell, which occur during normal operation, present yet another problem.
If a SOFC system is run on hydrocarbons, there will inevitably occur deposits and contaminations in the reformer and also at the anode of the SOFC, which will degrade the performance of the SOFC and the reformer. The most important deposits and contaminates are carbon, which arises due to the thermodynamics of chemical equilibrium depending on temperature, and sulphur, which is contained in the fuel, and higher hydrocarbons, which were not completely cracked in the reformer. All of these substances form deposits on catalytic functional layers in the reformer or on the SOFC anode and thus cover active surfaces, which will result in power loss. In addition, these deposits can embed themselves in ceramic layers, changing the structure and composition of material. Since the generation of the substances cannot completely be avoided, the problem primarily is that of removing them from the system.
From DE 10 2007 033 150 A1 a fuel cell system is known which is used for power generation in an automotive vehicle. The fuel cell system comprises a reformer, which generates a combustion gas from a fuel containing hydrogen (e.g. diesel) and from an oxidator furnishing oxygen (e.g. air), and at least one fuel cell, which is supplied with combustion gas at the anode and with air as cathode gas, for instance. During normal operation the reformer is fed fuel and oxidator. During regeneration operation the reformer is initially only fed anode exhaust gas coming from the fuel cell, and if later on an oxidator is added care should be taken that no oxygen enters the anode side of the fuel cell.
It is an object of the present invention to improve a method of operating a high-temperature fuel cell in such a way that the deficiencies described above will be avoided during standby operation, while the problem of removing unavoidable deposits and contaminations of the fuel cell will also be solved.